docs: def init_default(required, default, optional_default):
      """
      Returns optional default if field is not required and
      default was not provided.
  
      :param bool required: whether the field is required in a given model.
      :param default: default provided by creator of field.
      :param optional_default: default for the data type if none provided.
      :return: default or optional default based on inputs
      """
      if not required and default == NOTHING:
          default = optional_default
  
      return default